RAM is the key resource. We have around 10 users logged in at any one time and will have up to 15 documents in memory. The main document is around 300M. So we are running quite well on 12G RAM on the server.

Have a look at your data model to see if you can optimise the application to reduce the memory footprint - For example, can you replace datetime fields with separate date & time fields. Can you reduce the number of unique values in some fields.

Also nowadays 8Gb is a very small amount of ram in a server - memory is cheap and a very effective upgrade.